A projector is a device for projecting an image onto an object such as a screen. When projecting an image, the projected image may become distorted into a trapezoidal shape depending on the tilt angle of the projector and the object. In order to resolve the trapezoidal distortion of the projected image, there are projectors provided with an image processing apparatus for correcting (deforming) the image to be projected in advance.
There are image processing apparatuses for correcting the image to be projected based on the distance between the projector and the object as well as the tilt angle of the projector and the object.
Patent document 1 discloses a technology of projecting plural object points forming a predetermined pattern onto a surface of a projection object, and detecting the boundary between the projection object and the background (contour of projection object) based on the distance to the object points, and correcting the projection image (image to be projected) to correspond to the boundary.
When projecting images with a projector, in many cases, images are continuously projected for a predetermined time period. If the object or the projector is moved while projecting the images, the projector measures the distance and the tilt angle again to correct the projected images.
With the technology disclosed in patent document 1, trapezoidal correction of the projected image can be performed based on the distance to the object. However, in order to measure the distance, there are cases where it is necessary to interrupt the projection of the images by the projector, and project a predetermined pattern again.    Patent Document 1: Japanese Laid-Open Patent Publication No. 2005-229415